The invention is in the field of aqueous chemistry, particularly ion exchange chemistry. Various aspects of the invention involve interactions between ion exchange resins and gold-bearing thiosulfate solutions.
One of the conventional methods for the extraction of gold from its ores requires a cyanidation process in which a finely ground gold-bearing ore is leached with a solution containing cyanide ion. In the presence of an oxidant, usually oxygen as supplied by air, gold is dissolved into the solution as a cyanide complex, generating a gold-bearing leachate. The recovery of gold from the leachate can then be effected by adsorption with activated carbon, for example, using the widely adopted carbon-in-pulp (CIP) process, or by cementation with zinc using the well known Merrill-Crowe process.
In an alternative process to cyanidation, gold may also be leached from ores or other gold-bearing materials with a thiosulfate solution in the presence of oxygen under alkaline conditions. Gold is thought to be brought into solution in the form of a gold thiosulfate complex according to the following reaction:
4Au+8S2O32xe2x88x92+O2+2H2Oxe2x86x924Au(S2O3)23xe2x88x92+4OHxe2x88x92
The thiosulfate leach may involve the addition of ammonia, as the ammonium thiosulfate salt, and copper, usually as the pentahydrated cupric sulfate salt, as catalysts. This process is for example described in U.S. Pat. No. 4,269,622 dated May 26, 1981, U.S. Pat. No. 4,369,061 dated Jan. 18, 1983 and U.S. Pat. No. 4,654,078 dated Mar. 31, 1987. It has been suggested that one of the advantages of the thiosulfate leach may be that it is particularly suitable for treating some of the refractory ores, for example carbonaceous ores and copper-gold ores, for which cyanidation is ineffective or uneconomical.
PCT application WO 00/65112 teaches the recovery of gold from an aqueous ammoniacal thiosulfate solution by liquid/liquid or liquid/solid extraction with an extraction reagent having either a guanidyl functionality or quaternary amine functionality mixed with a weak organic acid. Gold is stripped from the extraction reagent with an aqueous caustic solution optionally containing cyanide. Thomas et al., U.S. Pat. No. 5,785,736 discloses recovery of gold and copper from a gold-bearing and copper-bearing thiosulfate solution using a suitable ion exchange resin to adsorb the metals onto the resin. The adsorbed copper is then eluted from the resin using a thiosulfate solution to generate a copper bearing eluate and the adsorbed gold is eluted using a thiocyanate solution to generate a gold bearing solution.
The present invention recognizes that the presence of tetrathionate, even at a very low concentration, may significantly decrease copper and gold loading on ion exchange resins. The present invention also recognizes that under typical conditions for thiosulfate leaching and ion exchange processes (such as pH 8-10 in the presence of copper), a large amount of tetrathionate may continuously be generated through copper catalyzed oxidation of thiosulfate by oxygen in the air. Consequently the adsorption of precious metals on resins may be inhibited and the resins may need to be reconditioned.
The present invention provides a process for loading gold onto an ion exchange resin having affinity for gold from a thiosulfate solution containing polythionates, such as tetrathionate, comprising reducing the concentration of polythionate, such as tetrathionate, in the gold-bearing thiosulfate solution prior to contacting the solution with an ion exchange resin. Reduction of tetrathionate concentration may for example be achieved by adjustment of the pH of the gold-bearing thiosulfate solution, such as by raising the pH to a value from about 10 to about 12, or by treatment at elevated temperature for a sufficient period of time, or by both pH and temperature adjustment.
In one aspect, the invention provides a process for loading gold onto an ion exchange resin from a thiosulfate solution in the presence of copper ions. The copper ions may be cupric ions, cuprous ions or mixture thereof. The cupric ions may comprise cupric sulfate, or cupric ammine or cupric sulfate and cupric ammine. The cuprous ions may comprise cuprous trithiosulfate, or cuprous dithiosulfate or, cuprous thiosulfate or mixtures thereof. The ion exchange resin may have affinity for both gold and copper. In one aspect, the process may comprise adjusting the pH or temperature or both pH and temperature of the gold-bearing and copper-bearing thiosulfate solution for a sufficient period of time to produce a gold-bearing and copper-bearing loading solution having a reduced tetrathionate concentration. The pH may for example be adjusted to a value from about 10 to about 12 and the temperature ranges from about 20xc2x0 C. to about 60xc2x0 C. In alternative embodiments the pH may be adjusted from a minimum of about 9.5, 10, 10.5, 11 or 11.5 to a maximum of 10, 10.5, 11, 11.5, 12 or 12.5, such as from about 10.5 to 11.5. In alternative embodiments, the pH may be about 9, 10, 11 or 12, with a pH of about 11 being preferred in some embodiments.
The invention also provides processes for recovering gold from copper-gold ores thiosulfate leaching or copper catalyzed gold ores thiosulfate leaching using ion exchange resin technology. In one aspect, the invention provides a process for recovering gold from a slurry comprising a solid ore residue and a gold-bearing and a copper-bearing thiosulfate solution containing tetrathionate comprising adjusting the pH and/or the temperature of the slurry for a sufficient period of time to produce a loading slurry having a gold-bearing and copper-bearing loading solution having a reduced tetrathionate concentration. The loading slurry may be contacted with an ion exchange resin having affinity for gold and copper to adsorb gold and copper from the gold-bearing and copper-bearing loading solution onto the resin to produce a loaded resin. Next the loaded resin may be separated from the slurry and copper and gold are eluted. In a preferred embodiment copper and gold are eluted with the same eluant and gold may be recovered from the gold-bearing and copper-bearing eluate by electrowinning or precipitation. Alternatively the gold-bearing and copper-bearing loading solution may be separated from the slurry and contacted with the ion exchange resin. Copper and gold are then eluted as previously described.
The decomposition of tetrathionate in alkaline solution gives thiosulfate and sulfite with most of the tetrathionate being transferred to thiosulfate which can be then advantageously recycled in the gold and/or copper leaching.
In another aspect, the invention provides processes for eluting gold and optionally copper loaded onto an ion exchange resin from a thiosulfate solution that are environmental compatible alternatives to the conventional processes using cyanide or thiocyanate as eluant. In one embodiment the elution process may be carried out with an eluant comprising a mixture of sodium tetrathionate, ammonium thiosulfate and sodium sulfite. The tetrathionate anions displace the gold-thiosulfate and optionally the copper-thiosulfate complexes from the resin with the thiosulfate stabilizing the eluted gold and optionally eluted copper and the sulfite preventing the precipitation of elemental sulfur. After elution the resin may be reconditioned by alkaline treatment to remove the adsorbed tetrathionate. The thiosulfate and sulfite resulting from the decomposition of the tetrathionate can be advantageously recycled in leaching process or in the elution stage of the recovery of gold and/or copper. In a preferred embodiment the eluant may be a mixture of 68 g/L of sodium tetrathionate, 75 g/L of ammonium thiosulfate and 32 g/L of sodium sulfite. Alternatively gold and optionally copper can be eluted from the resin with sodium sulfite or a mixture of sodium chloride and ammonium thiosulfate as the eluant, or a mixture of sodium sulfite and ammonium thiosulfate.